Surface emitting lasers (SELs), which generate light in a direction perpendicular to their epitaxial structure, have several advantages compared to conventional semiconductor edge-emitting lasers (EELs). For example, the light beam of a SEL is circular and does not have a significant degree of astigmatism as compared to the beam of an EEL. As a result, the light from a SEL can be effectively coupled with optical fibers without any additional optical elements, which can be important for optical data communication. For the same reason, it can be focused into the small diffraction-limited spot that is necessary for optical data storage applications. Good beam quality of SEL light is also advantageous for the efficient conversion of light into the double-frequency spectral range with the use of nonlinear crystals.
By emitting light perpendicular to its wafer surface, a SEL allows for a relatively easy production of compact arrays of many lasers on the stage of wafer processing and before packaging. In this way, the problem of the relative alignment of elements is eliminated. Thanks to surface emission rather than edge emission, SELs can be tested on-chip before the wafer is diced up and packaged into individual components. With this, bad devices can be thrown out at an early stage of processing thereby cutting processing cost per wafer by up to 80% relative to an edge-emitting wafer.
SELs are generally represented by VCSELs (Vertical Cavity Surface Emitting Lasers). In VCSELs, the length of the laser cavity is limited by the thickness of its epitaxial structure and does not exceed a few wavelengths. To provide an emitted beam in only a single fundamental spatial mode, the diameter of the beam should be limited as well (by some microns). Therefore, the diameter of the active medium in the laser structure has to be limited also, which leads to a low level of output of VCSEL power (usually less than 10 mW). Larger area VCSEL emitters, with beam diameters on the order of 100 μm, can produce output beams having a few hundred mW of CW output power. However, operation of conventional VCSELs at high power and large diameter generally carries with it the penalty of an output beam having high-order spatial modes and multiple frequencies.
Another class of SELs, Vertical External Cavity Surface Emitting Lasers (VECSELs), are still under development. The main idea included in the VECSEL design is the extension of the laser cavity by the use of an external dielectric mirror as an output coupler. In this way, the diameter of the fundamental laser beam can be proportionally increased with the corresponding increase of the active laser medium diameter. Therefore, higher optical energy can be collected in the laser beam under proper pumping conditions. The strict requirement for the media pumping to produce the single-mode beam is the uniformity of optical gain distribution in the active zone.
Attempts to provide direct electrical pumping of the broad area of active layers in SEL structures have failed because of problems with uniformity of current distribution. Therefore, VECSELs are generally represented at this time by optically pumped SELs where the light of additional semiconductor lasers is absorbed by the active region of the SEL and optical gain is generated there. Because the gain distribution reflects the intensity profile of external laser beams, its uniformity is easily achieved in this way.
However, there are two main drawbacks of optically-pumped SELs. First, the plug-to-light efficiency is not so high as for VCSELs. This is the case because two stages of energy conversion are included. Furthermore, such devices are necessarily large as a result of extra lasers that are incorporated therein.
Recently, a new approach to VECSEL making was proposed as is described in International Publication Number WO 98/43329 to Mooradian for High Power Laser Devices. The device in that disclosure combines two kinds of pumping. In a first step, the current activates the central part of the active media. Then, the peripheral part of the media is pumped by photons generated during the first step.
As shown in the cross-section of FIG. 1, the device contains a SEL chip with gain layers 6 and reflective layers 4 grown epitaxially on the substrate 8. A circular contact or electrode 2 and an annular contact or electrode 10 are deposited on the opposite faces of the wafer. The resonant cavity is provided by a mirror stack 4 and an external mirror 12. The device generates output light 16. The gain area is pumped electrically thereby causing current to flow between annular contact 10 and circular contact 2. The resulting current flow 14 is generally conical in shape, with the base of the cone being at or near the annular contact 10 and the peak of the cone being at or near the circular contact 2. The current flow energizes a central part of the gain region with a diameter D1. The diameter D1 should be substantially larger than the thickness of the gain region 6.
The excited gain region of diameter D1 generates stimulated and spontaneous emission, which generally travels in any direction relative to the propagation of the cavity laser beam. However, since the transverse gain length is larger than the longitudinal gain length, more stimulated emission can occur in that direction. This transverse energy is absorbed in a second annular volume, which surrounds the first pumped volume. This absorbed energy serves to pump the second volume providing gain and, therefore, power into the fundamental laser mode with a diameter D2. The size of diameter D2 is determined by the position and shape of external mirror 12.
A top view of the gain media 6 is shown in FIG. 2 where the central part 22 is pumped by current and provides the optical secondary pumping of annular region 20 by photons designated with arrows. The gain distribution over the area with diameter D2 is shown in the inset.
The abovedescribed prior art design resolved two problems that previously hindered the fabrication of high-power surface emitting lasers. First, the use of secondary optical pumping provides more uniform gain distribution than previously used attempts to activate bigger areas with current. As shown in FIG. 2, the flow of photons generated by current within the central area is averaged on angle and distance to the central area. In this way, the distribution of gain in the surrounding area becomes smooth and axially symmetrical thereby producing a single-mode output beam with high efficiency. Second, the numerous photons generated along the gain layer direction are not lost as in conventional SEL designs. Instead, they are used for secondary optical pumping of active media. Therefore, the efficiency of current-to-light conversion is increased.
However, there are some technical hurdles left unmet by the previously described device that limit its applications. One can see in FIG. 1 that the growth substrate 8 is the integral part of the design. It should be transparent to laser light. In most cases, however, the substrate material does not meet this requirement. As an example, GaAs is not transparent for light with a wavelength shorter than 870 nm, but it serves as a substrate growth for the epitaxial structure of SELs emitting light in the spectral range (650-850 nm).
As shown in FIG. 2, the radial gain distribution in the active media is still far from uniform. Also, the gain falls down at the borders of area with diameter D2. This is connected with electrical pumping of only the central part of the gain region.
An additional technical problem is connected with the necessity that the circular electrode 2 have a small size (<100 micron), which in turn should not contain any contacting electrodes disturbing its circular symmetry. Otherwise, the shape of current flow within the gain area will not be conical, and the output of the single-mode laser beam is decreased.
For these and further reasons, there remains a need for improvements in the art.